As explained in Torffield, U.S. Pat. No. 4,190,739, sound reproduction systems using passive, reflective surfaces in addition to active sound sources such as speakers are not unknown. The terms "speaker" and "oudspeaker" mean either a single sound source or a system of several sources, each contributing a part of the audible frequency range. In the past, individual speaker enclosures have employed reflective elements internally to direct sound in one direction or another. Speakers have been suggested for directing sound against nearby structural walls to cause reflection of the sound back towards a listening area. Sounding boards are known for use by instrumentalists to project sound either towards an audience's listening area or towards studio microphones. Various architectural acoustic elements have been employed or suggested for enhancing the acoustical characteristics of auditoriums.
In high fidelity sound reproduction systems however, the use of non-architectural sound reflectors spaced from a speaker to redirect sound towards a listener has been relatively unexploited. The loudspeaker industry has, for example, made attempts over several years to provide the listener with a kind of sound experience otherwise attainable only under real-life conditions. The familiar stereophonic loudspeaker systems have been commercially successful because, in comparison with monaural loudspeakers, sterophonic systems went a long way towards providing realistic sound experiences. Insofar as is known however, the prior art has not been able to provide a system which subjects a listener to the feeling of acoustical space, depth and scale which, in an almost inexplicable way, characterizes real-life live sound.
Such attempts have included sterophonic loudspeaker systems with two identical speakers pointed toward convexly curved surfaces for purposes of distributing the sound while eliminating the well known "beaming" effect inherent in sound emanating from speakers more or less equivalent to point sources. An example of such attempts is disclosed in Ranger, U.S. Pat. No. 3,065,816. Reflective arrangements have been proposed for low frequency sound components; it is generally conceded however, that such arrangements have produced an undesirable, blurred sound reproduction.
An interesting example of the use of reflection in an attempt to provide more realistic sound is shown by Karlson, U.S. Pat. No. 2,896,736 which proposes the use of a loudspeaker in an especially designed enclosure. In Karlson, a loudspeaker is pointed towards a wall to obtain greater angular dispersion of the sound than the typical ninety degree to one hundred and twenty degree sound dispersion characteristic of conventional conical loudspeakers radiating directly into an air space. Karlson suggests the use of curved surfaces to permit projection of sound over considerable distanced with minimal losses by reflecting the sound emanating from the specially enclosed loudspeaker via elliptical, hyperbolic and other curved surfaces.
The quality of reproduction which a loudspeaker provides within a room is known to be influenced by the interaction of the loudspeaker with adjacent reflecting boundary surfaces of the room, and especially by the sound reflected from any wall behind the loudspeaker. Sound reflected from such a wall arrives substantially in phase at very low frequencies, thereby increasing the efficiency of the loudspeaker within the low frequency range. The interaction of the sound emanating from the loudspeaker with adjacent walls however, causes an uneven frequency response known as the "comb filter effect." This effect occurs because the phase angle between the direct sound emanating from the loudspeaker and the sound reflected from the adjacent walls of the room is proportional to frequency, and the resulting sound at higher frequencies will alternate between minimum and maximum amplitudes. The first minimum usually occurs between 100 and 300 Hertz, depending upon the distance of the sound radiating surfaces from the wall. The reflected sound component distorts not only the amplitude response of the loudspeakers in the frequency domain, but also their amplitude response in the time domain.
Although roughened or irregular surfaces have been used in the acoustic arts, little attention has been given to the effects of the surface of a reflector. Wherever roughened or irregular surfaces have been used, such surfaces have been associated with sound absorption, rather than with sound reflection. The dispersive effect of roughened or irregular surface textures has been largely ignored in sound reproductive systems. In the reproduction of sound in currently available systems, a recurring phenomena has been differences in the angles of dispersion of the sound at various frequencies across the audible spectrum. Ordinarily, bass frequencies are more widely dispersed while the higher audio frequencies are more narrowly dispersed. This characteristic is called "beaming" by virtue of the narrower or beam-like projection of cones at higher frequencies.